The electric power steering system is designed to apply a steering assist torque to a steering mechanism by driving an electric motor based on a steering torque applied by a driver to a handle (steering wheel). More specifically, the electric power steering system provides a current control (feedback control) such that a target current may flow through the electric motor for generating the steering assist torque, the target current defined based on a steering torque detected by a torque sensor.
The conventional electric power steering systems include one which is provided with a stability compensator at a next portion of the torque sensor in order to improve the stability of a control system. Such an electric power steering system is disclosed in, for example, Japanese Unexamined Patent Publication No. H8-290778. The stability compensator of the conventional system has a characteristic expression G(s)=(s2+a1s+a2)/(s2+b2s+b2), where s denotes a Laplace operator and a1, a2, a3, a4 denote parameters decided by resonant frequencies of a resonant system.
In Japanese Unexamined Patent Publication No. 2004-216952, the present applicant has also proposed an electric power steering system which includes phase compensating means of the same type as a band-stop filter having a transfer function Gc(s) in order to serve a dual purpose of ensuring the stability as the control system and improving response. The above transfer function Gc(s) is represented by an expression Gc(s)=(s2+2ζ2+2ω2s+ω22)/(s2+2ζ1ω1s+ω12), where ζ1 denotes a compensated damping coefficient; ζ2 denotes a damping coefficient of a compensated system; ω1 denotes a compensated natural angular frequency; and ω2 denotes a natural angular frequency of the compensated system. The transfer function Gc(s) is set to a value such as to reduce or negate a peak of a gain characteristic of an open-loop transfer function for torque of the electric power steering system, the peak appearing based on natural vibrations of a mechanical system and a counter-electromotive force of the motor.
For example, a Bode diagram of the above phase compensating means is shown in FIG. 7. As apparent from a phase characteristic indicated by a solid line in FIG. 7, the above phase compensating means is capable of enhancing the stability of the system because the phase compensating means is adapted for phase advance in a frequency region higher than 20 Hz where the electric power steering system is decreased in stability margin due to the decline of the phase characteristic thereof.
The phase compensating means represented by the transfer function Gc(s) is of the same type as the band-stop filter. As seen from a gain characteristic indicated by the solid line in FIG. 7, therefore, the phase compensating means is capable of suppressing resonance (resonant frequencies: 10 to 30 Hz; in proximity of ‘A’ in FIG. 7) occurring in the system.
However, if the phase compensating means is excessively enhanced in the effect to suppress the resonance, the phase compensating means is decreased in gain in a normal steering speed region (frequency region in proximity of ‘B’ in FIG. 7) and also suffers an increased response lag in providing the assist (steering assist). Thus, steering feeling is adversely affected.
Specifically, the following problem occurs if the phase compensating means has an increased gain at the resonant frequencies on the negative-sign side in order to suppress the resonance. That is, the gain assumes negative values not only in proximity of the resonant frequencies but also in a broad frequency region about the resonant frequencies (a dot-dash gain characteristic curve in FIG. 7). What is more, the phase advance is increased in a frequency region higher than the resonant frequencies (in proximity of ‘A’ in FIG. 7), whereas the phase lag is increased in a frequency region (in proximity of ‘B’ in FIG. 7) lower than the resonant frequencies (a dot-dash phase characteristic curve in FIG. 7).
In order to minimize the response lag in the normal steering speed region B and to decrease the gain at the resonant frequencies, it may be contemplated to define the transfer function Gc(s) of the phase compensating means to satisfy ζ2<ζ1<1.
However, this approach cannot exclude floating-point arithmetic which poses a great arithmetic load on a controller (microcomputer). Hence, this approach is impracticable in the electric power steering system required of high-speed processing.
As described above, the transfer function Gc(s) may sometimes be unable to serve a dual purpose of suppressing the resonance and providing a good assist response in the normal steering speed region. Furthermore, an attempt to attain another desired characteristic encounters difficulties of reducing the arithmetic load and providing design freedom.
It is therefore an object of the invention to provide the freedom in designing a filter such as a compensator and also to obviate the increase of arithmetic load, thereby serving a dual purpose of, for example, suppressing the resonance and improving the assist response in the normal steering speed region.